Inverters have numerous applications in electrical power supplies including the production of alternating current power supplies, for example, when used as an inverter to convert a DC voltage into an AC voltage supply (e.g. an uninterruptible power supply). They may also be used in internal stages of DC to DC converters, induction heating, microwave generation, surface detection, medical experimentation, high frequency radio systems, IPT systems, etc.
A circuit schematic for a known push-pull current fed resonant inverter is shown in FIG. 1. The operation of such inverters is discussed in U.S. Pat. No. 5,450,305 the contents which are incorporated herein by reference. These resonant inverters have gained much popularity due to their low switching losses and low electromagnetic interference (EMI).
In IPT Systems the IPT power supply is ideally a fixed frequency power supply producing a fixed frequency sinusoidal output voltage. Such a power supply is shown in FIG. 1. The circuit of FIG. 1 has a DC inductor LDC, a split phase transformer LPS, and a parallel resonant tank circuit C1 L1. Switches S1 and S2 operate in anti-phase to produce a resonant voltage across the parallel tuned tank circuit. Diodes in series with the switches are added so that there is no possibility of the switches turning on at the same time and discharging C1.
The inductor LDC provides a constant DC current source under steady state operating conditions. This inductor is usually designed to be large to overcome saturation problems. The phase splitting transformer with the two closely coupled windings LSP is used to divide the DC current into two branches, and the switches S1 and S2 are controlled to be “on” and “off” alternately, to change the direction of the current that is injected into the resonant tank circuit which comprises the coil L1 and its tuning capacitor C1. The resistor R represents the load supplied by the inverter.
An external controller (not shown) is also required in order to control the switches S1 and S2. The controller detects the resonant voltage (for example sensing the voltage across tuning capacitor C1) and drives the switches at zero voltage crossings (Zero Voltage Switching). These switching techniques help to reduce the switching losses and EMI. To do so, an extra voltage transformer or winding is usually needed to detect the zero voltage crossings across the capacitor C1. The detected information is used by the controller to drive the switches S1 and S2 and special gate drive circuits are usually required. The start-up of this form of inverter is relatively difficult, requiring a complex controller.
When operating at high frequencies a conventional power supply (not shown) has problems since as the frequency gets higher it becomes increasingly difficult to operate the supply as the required dV/dt and dl/dt transients are so high that operating the power supply is problematic. For example at a frequency of 140 kHz a complete cycle is only 7 microseconds so that if the switches switch on a 480V bus in 1% of a half cycle then the dV/dt on the switches is 480/(3.5 microseconds×1%)=13.7 kV/microsecond which is a very fast transient that makes the operation of high-side switches challenging. The circuit of FIG. 1 avoids this problem with soft switching and low dl/dt and dV/dt. However a difficulty with this circuit (FIG. 1) is maintaining an operating frequency in response to changes in the reactive load on the inverter and in particular maintaining a required power factor since the circuit can easily bifurcate.